1. Field of the Invention
The present invention relates to an optical film and a polarizing plate each contributing to improvement in the contrast ratio, luminance and display unevenness of a liquid crystal display device, and a liquid crystal display device using the same.
2. Description of the Related Art
A liquid crystal display device is being widely used as a display device of various information processing units including a computer and a television. In particular, a TFT-type liquid crystal display device (hereinafter, sometimes referred to as “TFT-LCD”) has been widespread, and its market is expected to further expand. This involves the requirement to more enhance the image quality. In the following, the present invention is described by taking TFT-LCD as an example but is not limited to TFT-LCD and is applicable to general liquid crystal display devices, for example, applicable to liquid crystal displays such as passive matrix type and plasma address type.
The mode most widely used so far in the TFT-LCD is a so-called TN mode in which a liquid crystal having positive dielectric anisotropy is horizontally aligned between mutually opposing substrates. On the other hand, there is known a VA-mode liquid crystal display device in which a liquid crystal having negative dielectric anisotropy is vertically aligned between mutually opposing substrates. In the VA-mode liquid crystal display device, liquid crystal molecules are aligned almost vertically to the substrate surfaces when no voltage is applied, and this keeps the liquid crystal cell from showing birefringence and optical rotation and allows light to pass through the liquid crystal cell substantially without causing a change in its polarization state. Accordingly, by sandwiching the liquid crystal cell between a pair of polarizers (linear polarizers) while arranging their absorption axes orthogonal to each other (sometimes referred to as “cross-Nicol polarizers”), a substantially perfect black display can be realized when no voltage is applied. During voltage application, liquid crystal molecules are tilted to be in nearly parallel to the substrates and the liquid cell shows large birefringence to give a white display. Accordingly, the VA-mode liquid crystal display can easily realize a very high contrast ratio.
In such a VA-mode liquid crystal device, when liquid crystal molecules are tilted in one direction during voltage application, the liquid crystal display device shows asymmetric viewing angle characteristics. To avoid this problem, there is widely used, for example, a so-called MVA mode (multi-domain VA mode), which is an alignment division-type VA mode where the tilt direction of liquid crystal molecules is divided into a plurality of directions by devising the pixel electrode structure or providing an alignment control member such as protrusion in a pixel. From the standpoint of maximizing the transmittance in a white display state, the axial azimuth of a polarizer is set to make an angle of 45° with the tilt azimuth of liquid crystal molecules during voltage application. This is because the transmittance when a birefringent medium is sandwiched between cross-Nicole polarizers is proportional to sin 2 (2α) where α (unit: rad) is an angle between the axis of the polarizer and the slow axis of the birefringent medium. In a typical MVA mode, the tilt azimuth of liquid crystal molecules may be divided into four domains of 45°, 135°, 225° and 315°. Also in such an MVA mode with four divided domains, Schlieren alignment or alignment in an unintended direction is often observed at a domain boundary or near the alignment control member, and this gives rise to a transmittance loss.
In order to solve such a problem, a VA mode using a circularly-polarizing plate is being studied (see, for example, JP-A-2002-40428 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-2003-207782, JP-A-2003-186017). According to this liquid crystal display device, the transmittance when a birefringent medium is sandwiched between right and left circularly-polarizing plates orthogonal to each other is independent of an angle made by the axis of the polarizer and the slow axis of the birefringent medium. Therefore, as long as the tilt of liquid crystal molecules can be controlled, a desired transmittance can be secured even if the tilt azimuth of liquid crystal molecules is not 45°, 135°, 225° and 315°. Accordingly, for example, liquid crystal molecules may be tilted at every azimuth by providing a circular protrusion at the center of a pixel, or liquid crystal molecules may be tilted at random azimuths without controlling the tilt azimuth at all. In the description of the present invention, the VA mode using a circularly-polarizing plate is sometimes referred to as a circularly-polarizing VA mode or a circularly-polarizing mode. On the other hand, the VA mode using a linearly-polarizing plate is sometimes referred to as a linearly-polarizing VA mode or a linearly-polarizing mode. Also, as is well known, the circularly-polarizing plate is typically composed of a combination of a linearly-polarizing plate and a λ/4 plate.
As for a normal circularly-polarizing VA-mode liquid crystal display device. there is room for improvement in the point that the contrast ratio at an oblique viewing angle is low and sufficient viewing angle characteristics cannot be obtained. In this respect, various techniques for improving the viewing angle characteristics by using a retardation film have been proposed (JP-A-2010-211230, Akira Sakai, et al., Novel Wide-View Circular Polarizers Using Negative and Positive AC Plates, SID 09 DIGEST, pp. 402-405 (2009)).
However, the methods described in JP-A-2010-211230, Akira Sakai, et al., Novel Wide-View Circular Polarizers Using Negative and Positive AC Plates, SID 09 DIGEST, pp. 402-405 (2009) require many lamination steps for producing an optical film so as to realize a wide viewing angle and high luminance, and this leads to a cumbersome production process and a high cost. Furthermore, there is a problem that the thickness difference between the optical film used on the viewing side and the optical film used on the backlight side is large and when the liquid crystal display device is continuously lighted, unevenness is readily generated.